Method for casting high ti content alloys

ABSTRACT

METHODS FOR FORMING AND TREATING ALLOYS CONTAINING (A) TI AND NI, (B) TI, NI AND CO, (C) TI AND CO, (D) TI, CO AND FE, AND (E) TI AND FE WHICH INCLUDE PREPARING THE ALLOYS BY INDUCTION MELTING TECHNIQUES AND PRUIFYING THE ALLOYS BY CONTACTING THE MOLTEN ALLOY WITH CARBON. THE ALLOYS ARE FURTHER CAST IN SUITABLE GRAPHITE MOLDS WITH THE SOLIDIFICATION PROCESS PROCEEDING FROM THE BOTTOM TOWARD THE TOP OF THE MOLD.

.U.S. Cl. 75-110 United States Patent O 3,679,394 METHOD FOR CASTINGHIGH Ti CONTENT ALLOYS William J. Buehler, Bethesda, Md., assignor tothe United States of America as represented by the Secretary of the NavyNo Drawing. Continuation-impart of application Ser. No. 493,940, Oct. 7,1965, now Patent No. 3,508,914, dated Apr. 28, 1970. This applicationNov. 24, 1969, Ser. No. 879,622

Int. Cl. (32% 53/00,- C22d 7/06 8 Claims ABSTRACT OF THE DISCLOSUREMethods for forming and treating alloys containing (a) Ti and Ni, (b)Ti, Ni and Co, (c) Ti and Co, (d) Ti, Co and Fe, and (e) Ti and Fe whichinclude preparing the alloys by induction melting techniques andpurifying the alloys by contacting the molten alloy with carbon. Thealloys are further cast in suitable graphite molds with thesolidification process proceeding from the bottom toward the top of themold.

CROSS-REFERENCE TO RELATED APPLICATIONS This application is acontinuation-in-part of applica tion Ser. No. 493,940 filed Oct. 7,1965, now US. Pat. No. 3,508,914, issued Apr. 28, 1970.

BACKGROUND OF THE INVENTION This invention pertains to the formation andtreatment of alloys and more particularly to the formation and treatmentof alloys comprising a highly reactive metal and at least one relativelyunreactive metal.

Many techniques and methods have been tried in an elfort to find aneffective means for forming alloys containing a highly reactive metal.For example, attempts to form alloys of the highly reactive metaltitanium by fusion techniques were not very successful due to the highreactivity of molten titanium with the interstitial elements, oxygen,hydrogen, nitrogen and carbon. Thus, the use of well known ceramiccrucibles, e.g. SiO A1 etc., for forming titanium alloys resulted in adisastrous contamination due to the interaction of the titanium and theoxygen of the crucible. The use of high purity dense graphite crucibleswas not much more successful since carbide impurities resulted from theinteraction between the titanium and carbon of the crucible. The use oftechniques such as special electrical induction fields to minimizegraphite contact and thus minimize carbide formation were a little moresuccessful, but they are difficult and costly.

As a result of the failure of fusion methods, the alloys of highlyreactive metals are generally prepared by aremelting techniques e.g.titanium-nickel alloys have been formed by both consumable andnon-consumable methods, employing a water-cooled copper crucible. Thesetechniques, however, suffer from the following disadvantages:

1) Composition control is difficult due to the lack of stirringcapability in the overall melt.

(2) Little or no opportunity exists for alloy purification duringmelting.

(3) Costly multiple arc-melting operations are necessary to promotechemical homogeneity in the cast ingots.

(4) Only limited cast shapes can be produced.

SUMMARY OF THE INVENTION Accordingly, it is an object of this inventionto provide a novel method for forming alloys comprising a highly3,679,394 Patented July 25, 1972 reactive metal and at least onerelatively unreactive metal.

It is another object to provide a novel method for producing the abovementioned alloys that promotes chemical homogeneity.

It is a further object to provide a novel method for forming and castingthe above mentioned alloys.

It is still a further object of this invention to provide a novel methodfor forming and casting alloys containing (a) Ti and Ni, (b) Ti, Ni andC0, (c) Ti and Co, (d) Ti, Co and Fe and (e) Ti and Fe.

It is a still further object of this invention to provide a method forforming and purifying metal alloys containing Ti and either (a) Ni andC0, (b) Co, (c) Co and Fe or ((1) Fe.

It is yet another object of this invention to remove oxygen impuritiesfrom high Ti content alloys.

These and other objects will be more readily apparent from reading thefollowing detailed description of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The method of this invention forforming the alloy generally comprises melting the relatively unreactivecomponent or components of the alloy in a container formed from a stablematerial, adding to and melting in the container the reactive metalcomponent and pouring the resulting alloy into a suitable mold forsolidification. The melting operation is performed in an inertatmosphere, i.e. any atmosphere that excludes atmospheric contaminantsincluding, for example, a vacuum, a rare gas such as argon, helium,etc.; and the like, with the use of a rare gas at atmospheric pressureor higher being preferred since it prevents the leakage of air into thesystem.

The container is generally formed from either thoria or magnesia withmagnesia being preferred due to its lower cost and less toxic nature. Itis to be understood, however, that the term container formed from astable material is not limited to containers wholly made from suchmaterials but also includes containers made from other materials thathave their inner surfaces coated with stable materials. The stablematerial used for forming the container generally has a purity of atleast about 97%, with at least about 99% being preferred, in order tolessen the possibilities of oxygen impurities resulting from theinteraction of the reactive metal component with the oxide impuritiesgenerally found in such materials.

The relatively unreactive material or materials are melted in thecontainer first since it has been found that the molten unreactivecomponent or components have a moderating effect upon the reactive metalcomponent which reduces metal-container interaction. Since this moderating effect is considerably reduced when the atomic ratio of reactivecomponent to unreactive component or components exceed about 2 to l, themethod of this invention is most effective on alloys that fall withinthis ratio.

The components are generally melted in a low frequency induction furnacesince such an operation has a good mixing effect which promotes chemicalhomogeneity. Although low frequency induction melting techniques arepreferred because they produce superior alloys at a lower cost, it is tobe understood that other melting techniques may be employed so long asthey are utilized in an inert atmosphere and the relatively unreactivecomponent or components are melted first in a container formed from astable material such as magnesia or thoria.

The method of this invention is generally performed, using anickel-titanium alloy as a representative example, by first placing adry clean magnesia or thoria crucible containing the desired weight ofnickel into a low frequency induction furnace having an inertatmosphere. The nickel is melted and the amount of titanium necessary togive the desired nickel-titanium ratio is charged from a charging chuteinto the molten nickel. The molten titanium and nickel are intimatelymixed in the crucible by the low frequency of the furnace and whenalloying is completed, the molten alloy is charged into a suitable moldfor solidification.

The method of this invention for removing oxygen impurities generallycomprises contacting the molten alloy with carbon under vacuum. Moreparticularly, the alloy may be purified either by induction melting thealloy under a vacuum of at least about 10 mm. in a carbon container,preferably in the form of high density graphite or by induction meltingthe alloy under a vacuum of at least 10* mm. in any suitable containerfollowed by the addition of carbon. Although the invention is not to belimited or bound by any theorotical reactions or equations, it isbelieved that the removal of oxygen impurities is effected, using anickel-titanium alloy as a representative example, by the followingreaction:

The purification effect of the carbon may be enhanced by adding to themolten alloy a metal that both forms an oxide having a high heat offormation and does not alloy with the metals present in the moltenalloy. Thus for example, an excess of calcium or magnesium metal (basedon the amount necessary to combine with the oxygen impurities present)is placed beneath the surface of the molten alloy in the graphitecontainer, said metal and oxygen combining to form an oxide which israked off as a slag. When the purification is completed the excess metalhaving a higher vapor pressure is removed by vacuum techniques.Alternatively, the purification may be promoted by bubbling hydrogeninto the molten alloy in the carbon crucible, said hydrogen combiningwith the oxygen impurity to form water vapor.

The alloy forming the purification techniques described herein may beperformed on any alloy comprising highly reactive and relativelyunreactive metal components. As representative examples of highlyreactive metal components there may be mentioned Group IV metals such ashafnium, zirconium, titanium, etc.; rare earth metals such as cerium,etc.; and the like. As representative examples of relatively unreactivemetal components there may be mentioned iron, cobalt, copper, indium,aluminum, nickel, gold, lead, and the like. The methods of thisinvention are particularly applicable to alloys of the formulas TiNi,,Coand Tico l e wherein X is a factor from to 1 and wherein the Ti contentof the alloy constitutes approximately 50 Atomic percent of the alloyand NiCo or C0,,Fe essentially constitute the remaining approximately 50Atomic percent of the alloy, and especially to the stiochiometricnickel-titanium alloy (53.5 to 56.5 weight percent nickel, the remainderessentially titanium). The nickel-titanium alloys are described in US.Pat. 3,174,851 granted Mar. 23, 1965 and the alloys represented by theformulas TiNi Co and TiCo Fe are described in application Ser. No.843,887, filed June 12, 1969, now U.S. Pat. No. 3,558,369, issued Jan.26, 1971, which is a continuation-in-part of application Ser. No.579,185, filed Sept. 9, 1966, now abandoned. The subject matter of allthese applications is hereby incorporated by reference.

There are a wide variety of embodiments incorporating the teachings ofthis invention which may be utilized in producing a cast shape of analloy comprising a highly reactive and relatively unreactive metal ormetals. One such embodiment comprises forming the alloy by the method ofthis invention, pouring the molten alloy before solidification into acarbon crucible, said alloy being kept molten under vacuum to effectpurification and pouring the molten alloy into a suitable mold forsolidification.

Alternatively, any suitable crucible may be used followed by theaddition of carbon to effect purification.

Another embodiment comprises placing an ingot prepared by either themethod of this invention or any other method into a graphite crucible,induction melting the alloy under vacuum to effect purification andpouring the molten alloy into a suitable mold for solidification.Alternatively, any suitable crucible may be employed followed by theaddition of carbon to the molten alloy.

A further embodiment comprises pouring the molten alloy prepared by themethod of this invention before solidification through a carbon linedfunnel into a suitable mold for solidification, said operation beingperformed under vacuum.

Still another embodiment for providing intricate cast shapes whileeffecting purification comprises delivering under vacuum the alloyformed by the method of this invention in either liquid or solid form toa specially designed mold made of high purity dense dried graphite. Themold has a suitably matched induction coil around it and the inductioncoil is equipped with taps to regulate heating specific sections of themold. If the charge to the mold is molten, the mold may be preheated toa temperature in excess of the melting point of the alloy concerned.Once the mold is filled (including the filling of generous hot top), theinduction coil is gradually closed off starting at the bottom of themold. By solidifying the casting from the bottom of the mold to the topit is possible to produce a minimum shrinkage pipe, and it will be inthe hot top section. Further, by casting in the heated mold it ispossible to provide a fine cast surface, optimize casting definition inthin sections and minimize porosity within the cast section. If thecharge to the mold is solid, the solid alloy is placed in a graphitehopper attached to the top of the mold, said induction coils being woundwell up on the hopper. The mold and hopper is heated by inductioncausing the alloy concerned to melt in the hopper and run into the mold.The solidification is then preformed as mentioned above placing theshrinkage pipe in the hopper.

It is to be understood that the term graphite mold is not limited tomolds wholly made of graphite but also includes containers made fromother materials that have their inner surfaces coated with graphite.

The following examples are illustrative of the invention but they arenot intended to limit it in any manner.

Example I A 55 Weight percent nickel, remainder essentially titaniumalloy may be prepared in the following manner.

2750 grams of nickel in the form of carbonyl nickel shot (99.9% pure)are placed in a magnesium oxide crucible (99.9% pure). The crucible isplaced in an induction furnace which has an atmosphere of argon at apressure of one atmosphere. The furnace is run at an induction input ofapproximately 3,000 cycles and the nickel in the crucible is heated to atemperature slightly in excess of 1600 C. in order to effect melting.2,250 grams of titanium (in the form of loose sponge) are added to themolten nickel and the temperature is maintained slightly in excess of1500 C. until the titanium and nickel are completely mixed (usually lessthan 5 minutes). The alloy melt is immediately poured into a suitablemold for solidification.

Example II A nickel-titanium alloy (55 weight percent nickel, remainderessentially titanium) is purified by placing the alloy in a high densitygraphite crucible which is then placed into an induction furnace. Avacuum of 10- millimeters is pulled and the induction input to thefurnace is approximately 3,000 cycles. The alloy is held at about 1500C. to effect purification and it is then poured into a mold forsolidification.

The same techniques hereinabove described can be applied to alloyscontaining (a) Ti and Ni, (b) Ti, Ni and Co, (c) Ti and Co, ((1) Ti, Coand Fe and (e) Ti and Fe wherein the Ti content of the alloys variesfrom 40 to 60 atomic percent and the other component or componentsessentially constitute the remainder of the alloy.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. In a method for forming and purifying a metal alloy,

selected from the group consisting of alloys comprising (a) Ti, Ni andCo,

(b) Ti and Co,

(c) Ti, Co and Fe, and

(d) Ti and Fe wherein the Ti content of the alloy varies from 40 to 60atomic percent of the alloy, said method comprising;

(1) induction melting the metals of the alloy in an induction meltingcontainer formed froma material selected from the group consisting ofmagnesia and thoria, said induction melting being performed in an inertatmosphere;

(2) contacting the molten alloy with carbon, in said induction meltingcontainer said contacting being performed in an inert atmosphere, and

(3) solidifying said alloy,

the improvement comprising first induction melting the non Ti componentand thereafter adding to and induction melting in the same container thetitanium component of the alloy.

2. The method of claim 1 wherein said inert atmosphere is a vacuum of atleast about mm.

3. The method of claim 2 wherein said contacting with carbon isperformed by adding carbon to the molten alloy.

4. The method of claim 2 wherein the container is formed from magnesia.

References Cited UNITED STATES PATENTS 2,548,897 4/1951 Kroll 102,806,271 9/ 1957 Operhall 7549 3,188,198 6/196'5 Moore 7549 2,707,6795/ 1 955 Lilliendahl 7 584 3,429,691 2/ 1969 McLaughlin 75--1l 2,874,9532/1959 Rogers 7584 2,702,239 2/ 1955 Gilbert 7584 3,183,078 5/1965Ohtake 7549 3,223,519 12/ 1965 Schippereit 75--l0 3,417,808 12/1968Rosenberg 164--68 WINSTON A. DOUGLAS, Primary Examiner P. D. ROSENBERG,Assistant Examiner US. Cl. X.R. 7584-

